prevent install error nokogiri on Ruby < 1.9.2

https://travis-ci.org/chiliproject/chiliproject/jobs/7918265
```
Gem::InstallError: nokogiri requires Ruby version >= 1.9.2.
An error occurred while installing nokogiri (1.6.0), and Bundler cannot continue.
Make sure that `gem install nokogiri -v '1.6.0'` succeeds before bundling.
```